Immune mechanisms in the prevention of intra-abdominal abscess formation.

Bacteroides fragilis is the most commonly isolated anaerobe from intraabdominal infections. In experimental models of intraabdominal sepsis, B. fragilis has been shown to be uniquely virulent. Some of these virulence traits are due to the capsular polysaccharide of this organism. Immunity to infection secondary to B. fragilis seems to involve both arms of the immune system. Humoral immunity (complement, antibody and PMNs) is critical to clearance of these bacteria from the blood stream. Cellular immune mechanisms predominate against intraabdominal abscess formation. Adoptive transfer experiments have shown that a CD8+, IJ+, non H2 restricted immune T cell or lysate from this T cell confers protection to immunocompetent, naive mice. An in vivo system has been developed to begin defining the mechanism of protection. B. fragilis placed inside a filter containment chamber within the peritoneum of immune mice are specifically killed over an 8-day period in the absence of white blood cells. This killing phenomenon was also observed inside filter chambers within mice receiving adoptively transferred immune T cells or lysates of these T cells. Furthermore, killing is specific to B. fragilis. These results support a T cell dependent mechanism for killing this bacteria and provide an interesting model for further exploration.